Temecula sits on Interstate 15 between San Diego (60 miles south) and Los Angeles/the Inland Empire (90 miles north). There is no commercial airport in town; most visitors fly into San Diego or Ontario and drive.
Avoid arriving on Friday afternoon or leaving on Sunday afternoon — the I-15 between Temecula and San Diego is notorious for weekend backups. Greyhound serves Temecula from San Diego and Riverside, but the city is built around cars and a rental is effectively mandatory if you want to visit the wineries.